Conventionally, spark plugs are used for ignition in internal combustion engines such as automobile engines. A typical spark plug has a central electrode that forms an electrode for spark discharge on its leading end side, an insulator that holds that central electrode in an axial hole and a metal housing that surrounds and holds the radial periphery of this insulator. Furthermore, the spark plug is mounted in the engine by screwing the threads formed on the outer peripheral surface of the metal housing into the female threads provided in a mounting hole in the engine head, and an air-fuel mixture is ignited by the spark discharge.
In recent years there has been progress in increasing the output and reducing the fuel consumption of internal combustion engines, and while the engines are getting smaller, there have been increases in the diameter of exhaust valves to improve exhaust efficiency and denser formation of the water jacket to improve cooling efficiency. The diameter of the mounting holes for spark plugs has been reduced to be able to assure freedom of design on the engine side, and for the spark plugs mounted in these mounting holes, small ones where the nominal diameter for the threads is 12 mm or less are used.
Furthermore, the thickness of the engine head has been reduced because of the reduction in size, and the same heat load has a larger effect at the periphery of the mounting hole where the spark plug is affixed than it did conventionally. This is because there has been a relative reduction in the heat capacity because of the reduction in the thickness of the engine head. Since the temperature on the combustion chamber side of the mounting hole is particularly high in the vicinity of the opening, the thermal stress caused by the combustion of the air-fuel mixture is large. In addition, the effects of the mechanical load that the spark plug applies to the mounting hole because of the screwing in of the threads are especially great, and there is a danger of damage in the vicinity of the opening on the combustion chamber side of the mounting hole. The engine head has become more susceptible to thermal stress overall in the area around the mounting hole that secures the spark plug. To solve this problem, it is good to eliminate or reduce the part where the spark plug is in close contact in the vicinity of the opening on the combustion chamber side of the mounting hole. Here, for example, with the reduction in the inside diameter of the mounting hole in the engine head for the purpose of increasing the freedom of engine design, there are spark plugs where no threads are formed on the outer peripheral surface on the leading end side and threads that are difficult to reduce in diameter are formed on the base end side of the outer peripheral surface of the metal housing (for example, Published Unexamined Patent Application No. JP 2003-308945. If this spark plug is used, the part where the mounting hole and spark plug are in close contact can be eliminated in the vicinity of the opening on the combustion chamber side of the mounting hole, and the effects of the thermal stress that may arise at the mounting hole can be mitigated.
By the way, the heat accompanying combustion of the air-fuel mixture in the combustion chamber that is taken up by the grounding electrode and metal housing mainly escapes via the threads. If no threads are formed on the outer peripheral surface on the leading end side of the metal housing, there is a reduction in the contact surface area of the metal housing and mounting hole. Therefore, there is a danger that sufficient heat conduction from the metal housing to the engine head will not occur, and the heat resistance of the spark plug will be reduced.
It is an object of the present invention to solve the problems described above and provide a spark plug with improved heat resistance and an internal combustion engine in which that spark plug is mounted where good heat conduction in the vicinity of the opening on the combustion chamber side of the mounting hole for the spark plug provided in the engine head occurs while mitigating the effects of thermal stress that may arise in the vicinity of that opening.